Method and system of image capture based on logarithmic conversion

ABSTRACT

Image capture based on logarithmic conversion. At least some of the illustrative embodiments are methods including capturing an image by: reading a plurality of analog signals from an array of light sensitive elements; performing logarithmic analog-to-digital conversion on the plurality of analog signals which creates a corresponding plurality of digital values; and storing representations of the plurality of digital values to a storage medium.

BACKGROUND

Since the advent of photography, in many cases the dynamic range of ascene's luminance that a photograph can capture has been significantlyless than the range of luminance perceived by human eyes directlyviewing the same scene. Adjustments to the shutter speed, aperture, andfilm “speed” may help adjust the exposure to adequately capture theportions of the scene (e.g., the person under the umbrella at thebeach), but other portions of the scene (e.g., in the beach example, theocean behind subject) may be over exposed and thus carry little, if any,detail. The limitation regarding dynamic range occurs not only withcameras that utilize film, but also digital cameras that store images asdigital files.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of example embodiments, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows a plot of luminance against digital luminance values;

FIG. 2 shows a plot of luminance against digital luminance values, andalso apparent brightness against digital luminance values;

FIG. 3 shows a plot of luminance against digital luminance values, andalso apparent brightness against digital luminance values;

FIG. 4 shows, in block diagram form, an imaging system in accordancewith at least some embodiments;

FIG. 5 shows a plot of luminance against digital luminance values, andalso apparent brightness against digital luminance values, in accordancewith at least some embodiments;

FIG. 6 shows a plot of digital luminance values against luminance (log)in accordance with at least some embodiments;

FIG. 7 shows, in block diagram form, an imaging system in accordancewith at least some embodiments;

FIG. 8 shows, in block diagram form, a portion of an imaging system inaccordance with at least some embodiments;

FIG. 9 shows, in block diagram form, an imaging system in accordancewith at least some embodiments;

FIG. 10 shows a plot of luminance against digital luminance values, andalso apparent brightness against digital luminance values, in accordancewith at least some embodiments; and

FIG. 11 shows a method in accordance with at least some embodiments.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, different companies may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function.

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . . ” Also, theterm “couple” or “couples” is intended to mean either an indirect ordirect connection. Thus, if a first device couples to a second device,that connection may be through a direct connection or through anindirect connection.

“Logarithmic analog-to-digital conversion” shall mean that, for aplurality of analog input signals converted, a respective plurality ofdigital values are produced where the digital values are logarithmicallyrelated to the respective plurality of analog input signals. Creating aplurality of digital values linearly related to analog input signals andlater modifying the plurality digital values to have a logarithmicrelationship shall not be considered “logarithmic analog-to-digitalconversion”.

“Logarithmic analog-to-digital converter” shall mean a device orcombination of devices that, for a plurality of analog input signalsapplied to the device(s), a respective plurality of digital values areproduced where the digital values are logarithmically related to therespective plurality of analog input signals. A device that creates adigital value linearly related to an analog input signal in combinationwith later modifying the digital values to a logarithmic relationshipshall not be considered a “logarithmic analog-to-digital converter”.Moreover, incremental changes inherent in binary representation of acontinuous function shall not obviate the status as logarithmicanalog-to-digital converter. Moreover, localized departure from alogarithmic response (e.g., upper end of a conversion range, lower end aconversion range, temperature dependent changes) shall not obviatestatus as logarithmic analog-to-digital converter.

“Linear analog-to-digital converter” shall mean a device or combinationof devices that produce a digital value that is linearly related ananalog input signal. Incremental changes inherent in binaryrepresentation of a continuous function shall not obviate the linearityof a linear analog-to-digital conversion. Moreover, localizednon-linearity (e.g., upper end of a conversion range, lower end aconversion range, temperature dependent changes) shall not obviatestatus as linear analog-to-digital conversion.

“Octave” shall mean a unit of measure corresponding to a range ofintensities (e.g., perceived brightness, voltage, or measured photons),but octave shall not imply any relationship to or number of steps of aset of numbers that may reside within the octave or the number ofoctaves over the entire range of interest.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure, including the claims. Inaddition, one skilled in the art will understand that the followingdescription has broad application, and the discussion of any embodimentis meant only to be exemplary of that embodiment, and not intended tointimate that the scope of the disclosure, including the claims, islimited to that embodiment.

The various embodiments are directed to imaging systems, such as mobiledevices (e.g., wireless network enabled devices), mobile cellulardevices, digital cameras, scanners, and copiers. The various embodimentswere developed in the context of digital cameras, and the descriptionthat follows is based on the developmental context; however, theconcepts described are equally applicable to other image capturedevices, and thus the developmental context shall not be read as alimitation as to the scope of the claims below. Moreover, so as not tounduly complicate the presentation of the information regarding thevarious example embodiments, initially the specification is limited toblack and white imaging and images.

Identifying Shortcomings of the Related-Art

Part of understanding why the example embodiments represent an advancein imaging system technology is an understanding of the shortcomings ofrelated-art systems. In particular, both film cameras and digitalcameras available prior to the current specification convert luminanceof a scene linearly. In the case of film cameras, photons of lightinteracting with light sensitive film elements linearly recordluminance. In the case of digital cameras the conversion of analogsignals from each pixel of an array of light sensitive elements is alinear conversion to a digital value. The balance of this discussionwill be limited to digital cameras with the understanding that the sameissues arise in the film camera context.

“Linear” in the context of the identifying the shortcomings ofrelated-art systems indicates that each digital representation of aluminance value is related to the analog signal created by each pixel ofan array of light sensitive elements in a straight line sense. That is,each digital value is related to the corresponding analog signalaccording to the equation:VALUE_(DIGITAL) =M*(ANALOG VALUE)+OFFSET  (1)where VALUE_(DIGITAL) is the encoded digital value representation, M isgain value, ANALOG VALUE is the analog signal created by a pixel in thearray of light sensitive elements, and OFFSET is an offset value.

Thus, digital representations of luminance in related-art systems arelinearly related to luminance of the scene. As an example, consider animaging system that uses an 8-bit linear analog-to-digital converter,such that digital luminance values for the illustrative system may spanthe binary range {00000000→11111111} which in decimal is {0→255} (toavoid divide by zero issues, apparent brightness will be considered tospan the decimal range {1→256}). Further consider that in an examplecaptured image the lowest luminance pixel in the captured image takesthe value 1 and the highest luminance pixel in the captured image takesthe value 256. Thus, using linear conversion of luminance the entireluminance range of the captured image may be considered to be dividedinto 256 equally space steps along the range, in this example.

FIG. 1 shows a graph of the relationship between luminance (Y-axis) andthe digital luminance values (X-axis) in this example situation. Inparticular, for any luminance value (Y-axis), the corresponding digitalluminance value (X-axis) is linearly related as shown. Consider, forpurposes of explanation, two luminance values, one being the minimumluminance (reference number 100 in the figure), and the second beingluminance (reference number 102 in the figure), separated by adifference (reference number 104 in the figure). The difference 104 asbetween the two illustrative luminance values 100 and 102 results in anincremental change (reference number 106 in the figure) in the digitalluminance value. In the illustrative case of 8 bit linearanalog-to-digital conversion, the example difference 104 results in achange of decimal 64 in the digital luminance value. Now consider thesame magnitude of difference in luminance at the upper end of theluminance spectrum. That is, consider two luminance values, one beingthe maximum luminance (reference number 108 in the figure), and thesecond being luminance (reference number 110 in the figure), separatedby a difference (reference number 112 in the figure) having the samemagnitude as difference 104. The difference 112 as between the twoillustrative luminance values 108 and 110 results in an incrementalchange 114 in the digital luminance value. In the illustrative case of8-bit linear analog-to-digital conversion, the example difference 112results in a change of decimal 64 in the digital luminance value (i.e.,the difference between 192 and 256). Given the linear relationshipbetween luminance and digital luminance values, and since the magnitudeof the differences 104 and 112 are the same in this example, so too arethe magnitudes of the incremental changes 106 and 114 within the digitalluminance values. The specification now turns to how humans perceivebrightness.

Humans perceive changes in brightness non-linearly, and the relationshipis approximately logarithmic. Consider, for example, a digital imagecreated using the 8-bit analog-to-digital conversion discussed above. Ifdigital luminance values are applied to a display device, humanperception views the difference of luminance from a value of decimal 1to decimal 2 as a doubling of brightness; however, the next doubling ofbrightness is decimal 4 (2*2), not a decimal 3 (2+1). Likewise, the nextdoubling of brightness is decimal 8 (4*2), and so on. On the upper endof the digital luminance values in the example, human perception viewsthe difference of luminance from a value of decimal 128 to decimal 256as a doubling brightness.

A bit more precisely then, human perception of change of brightnessfollows the Weber-Fechner law for apparent brightness, definedmathematically as:B=k*ln(L/L ₀)  (2)where B is the change in apparent brightness to a human observer, k is aconstant, L is the just-observable change in luminance, and L₀ is thebackground luminance (or, previous luminance). Letting k equal 1, andapplying the values described above with respect to the systemperforming 8-bit analog-to-digital conversion, the change in apparentbrightness as between digital luminance values of decimal 1 and decimal2 is B=ln(2/1)=0.693. Again with k equal 1, the change in apparentbrightness as between digital luminance values of decimal 128 anddecimal 256 is B=ln(256/128)=0.693. The precise value 0.693 is of littleconsequence, but note that the change in apparent brightness in the twoexample situations is exactly the same in spite of the number of stepsbetween the respective analyzed values.

The inventor of the present specification has found that the linearanalog-to-digital conversion used in related-art imaging systemsconsidered with the human's logarithmic perception of brightnessdegrades image quality by storing or recording too much information inthe higher luminance ranges, and too little information in the lowerluminance ranges. In order to highlight this point, FIG. 2 is presented,which helps graphically illustrate at least some shortcomings ofrelated-art systems. In particular, FIG. 2 relates a luminance range(Y-axis on the left) against the range of possible digital luminancevalues (X-axis, illustratively ranging from 1 to 256) by way of solidline 200, where the relationship is linear. Co-plotted on the same graphis human perception of brightness (Y-axis on the right (with anarbitrary scale of 0 to 100%)) with respect to the digital luminancevalues (again, the X-axis) by way of dash-dot-dash line 202. Thus, aswith respect to FIG. 1, FIG. 2 illustrates a linear relationship betweenluminance and the digital luminance values by way of line 200. However,FIG. 2 also shows the human perception of brightness by way of line 202against the range of possible digital luminance values, and that therelationship between the digital luminance values and the apparentbrightness is non-linear, and in fact is logarithmic.

By performing linear analog-to-digital conversion, the related-artimaging systems store too much information with respect to the higherluminance ranges, and too little information with respect to the lowerluminance ranges. Again, human perception views the change in luminancebetween a decimal 128 value and a decimal 256 value as doubling ofbrightness. Thus, as shown in the illustrative case of FIG. 2, half thebit width of the digital luminance values in this example populates therange of the highest 12.5% (87.5% to 100%) of the apparentbrightness—half the bit width encodes changes in what a human mayperceive as pure white. The remaining 128 decimal values—the lower halfthe bit width of the digital luminance values in this example—populatesthe range of the lowest 87.5% of the apparent brightness.

Stated in terms of granularity or quantization between doubling ofapparent brightness, the difference in brightness as perceived by ahuman of a pixel having a decimal 128 value and a pixel having a decimal256 value will be a doubling of apparent brightness, in this case with128 gradations between. The difference in brightness as perceived by ahuman of a pixel having a decimal 64 value and a pixel having a decimal128 value will be a doubling of apparent brightness, with 64 gradationsbetween. The difference in brightness as perceived by a human of a pixelhaving a decimal 32 value and a pixel having a decimal 64 value will bea doubling of apparent brightness, with 32 gradations between. Thedifference in brightness as perceived by a human of a pixel having adecimal 16 value and a pixel having a decimal 32 value will be adoubling of apparent brightness, with 16 gradations between. Thedifference in brightness as perceived by a human of a pixel having adecimal 8 value and a pixel having a decimal 16 value will be a doublingof apparent brightness, with 8 gradations between. The difference inbrightness as perceived by a human of a pixel having a decimal 4 valueand a pixel having a decimal 8 value will be a doubling of apparentbrightness, with 4 gradations between. The difference in brightness asperceived by a human of a pixel having a decimal 2 value and a pixelhaving a decimal 4 value will be a doubling of apparent brightness, with2 gradations between. Finally, the difference in brightness as perceivedby a human of a pixel having a decimal 1 value and a pixel having adecimal 2 value will be still be a doubling of apparent brightness.

The information discussed in the immediately previous paragraph isreproduced in the table form below:

TABLE 1 START VALUE STOP VALUE QUANTIZATION 1 2 1 2 4 2 4 8 4 8 16 8 1632 16 32 64 32 64 128 64 128 256 128 Total 256Notice how the granularity within each doubling of brightness getssmaller at the lower luminance range.

In order to help quantify the difference between the related-art systemsand the various example embodiments discussed below, the range ofapparent brightness in FIG. 2 is logically divided into eight equallyspaced divisions or “octaves” (labeled octave 204, 206, 208, 210, 212,214, 216, and 218) where the divisions are based on doubling in apparentbrightness. While in the example situation of an 8-bit linearanalog-to-digital conversion nicely divides into eight octaves, anynumber of octaves may be chosen (e.g., ten octaves). Thus, octave 218represents the highest or brightest octave, and octave 204 representsthe lowest or most dim octave.

While each illustrative octave represents a doubling of apparentbrightness, FIG. 2 also shows that the granularity or quantizationwithin each octave is overloaded in the upper octaves. Thus, forexample, pixels in octave 218 can have any of 128 possible digitalluminance values. Pixels in octave 216 can have any of 64 possibledigital luminance values. Jumping to the lowest octave 204, pixels inoctave 204 can have only a value of decimal 1 or a value of decimal 2.Thus, even though the human eye can perceive a great number of shades ofgray (or color) within each octave (e.g., 512 or more), very littleinformation is stored in the lower octaves in comparison to the upperoctaves. Table 2 immediately below is similar to Table 1 above, but alsoincludes the octave information (the reference number for each octaveshown parenthetically).

TABLE 2 OCTAVE START VALUE STOP VALUE QUANTIZATION 1 (204) 1 2 1 2 (206)2 4 2 3 (208) 4 8 4 4 (210) 8 16 8 5 (212) 16 32 16 6 (214) 32 64 32 7(216) 64 128 64 8 (218) 128 256 128 Total 256Thus, too much information is encoded with respect to the upperluminance ranges (the upper octaves), and too little is encoded withrespect to the lower luminance ranges (the lower octaves).

Wider A/D Conversion does not Address the Problem

One approach in the imaging system industry to obtain higher resolutionimages is merely to use a “wider” linear analog-to-digital conversion.For example, rather than use an 8-bit linear analog-to-digitalconversion, one approach in the imaging industry is use of 12-bit linearanalog-to-digital conversion, or perhaps a 16-bit linearanalog-to-digital conversion. However, even a wider linearanalog-to-digital conversion does not fully address the issue.

Assume for purposes of explanation that a digital camera manufacturerwishing to store data with greater resolution modifies a camera systemto have a 16-bit linear analog-to-digital conversion rather than the8-bit linear analog-to-digital conversion used in the example above.There are a host of problems that would dissuade a camera manufacturerfrom making a switch to a 16-bit linear analog-to-digital conversion,not the least of which are increased price, increased power requirementsresulting in shorter battery life, as well as serious questions as towhether 16-bit linear analog-to-digital conversion could operate quicklyenough to be used in a camera device. Notwithstanding these issues,consider FIG. 3, which is similar to FIG. 2, except based on use of a16-bit linear analog-to-digital conversion.

In particular, FIG. 3 shows luminance range (Y-axis on the left) againstthe range of possible digital luminance values (X-axis, illustrativelyranging from 1 to 65,536) by way of solid line 300, where therelationship is linear. Co-plotted on the same graph is human perceptionof brightness (Y-axis on the right (with an arbitrary scale of 0 to100%)) with respect to the digital luminance values (again, the X-axis)by way of dash-dot-dash line 302. Thus, as with respect to FIG. 2, FIG.3 illustrates a linear relationship between luminance and the digitalluminance values by way of line 300. However, FIG. 3 also shows thehuman perception of brightness by way of line 302 against the range ofpossible digital luminance values, and again that the relationshipbetween the digital luminance values and the apparent brightness isnon-linear.

For the same luminance range as between FIG. 2 and FIG. 3, the humanperception of brightness will not change. Thus, line 302 is the same asin FIG. 2, and the octave assignments (along the right Y-axis) remainthe same. It follows that, under these assumptions, human perceptionviews the change in luminance between a decimal 32,768 value and adecimal 65,536 value as doubling of brightness. As shown in theillustrative case of FIG. 3, half the bit width of the digital luminancevalues in this example (32,768 decimal values) populates upper mostoctave 218. Stated in terms of granularity or quantization betweendoubling of apparent brightness, the difference in brightness asperceived by a human of a pixel having a decimal 32,768 value and apixel having a decimal 65,536 value (octave 218) in this example will bea doubling of apparent brightness, in this case with 32,768 gradationsbetween. The difference in brightness, as perceived by a human, of apixel having a decimal 16,384 value and a pixel having a decimal 37,768value (octave 216) will be a doubling of apparent brightness, with16,384 gradations between. Skipping to the lower most octave, thedifference in brightness as perceived by a human of a pixel having adecimal 1 value and a pixel having a decimal 2 value (octave 204) willbe a doubling of apparent brightness. Thus, even if 16-bit linearanalog-to-digital conversion could be used in the example camera system,the 16 bit conversion still overloads storage in the upper octaves, andstores too little information in the lower octaves. No amount ofpost-processing can recover information for illustrative octave 204—theinformation is simply not recorded.

One additional point is made with reference to FIG. 3, which ties inwith aspects later presented. The mathematical center of the digitalluminance values in linear analog-to-digital conversion is not thecenter of the apparent brightness scale—the mathematical center is inthe upper octaves of the apparent brightness. For example, if oneassumes that for a particular image captured the lowest digitalluminance value is a decimal 1, and the highest digital luminance valueis a decimal 65,536, the average of the two values is about 32,768. Forthe example situation of eight zones and the image captured spanning theentire dynamic range of the linear analog-to-digital conversion, theaverage or mathematical center of the digital luminance values is notthe center or midpoint of the apparent brightness. Rather, the example32,768 average value resides at or very near the upper most octave 218.

Example Embodiments

The issues noted above are addressed, at least in part, by use oflogarithmic analog-to-digital conversion. FIG. 4 shows, in block diagramform, a system in accordance with at least some embodiments. Inparticular, FIG. 4 shows an imaging system 400, which may beillustrative of any device which captures and stores images, such asmobile cellular device, a digital camera, a scanner, and a copier. Theimaging system 400 comprises an array of light sensitive elements 402coupled to a logarithmic analog-to-digital conversion system 404 by wayof a selective coupling network 406. The imaging system furthercomprises processor 408 coupled to a program memory 410 and an imagememory 412 by way of a communication bus 414. In cases where the imagingsystem 400 is a portable device, the imaging system 400 may furthercomprise a battery 416. The electrical connections of the battery 416 tothe various other components of the imaging system 400 are omitted so asnot to unduly complicate the figure.

The array of light sensitive elements 402 may take any suitable form forcapturing representations of a scene or image, such as the illustrativeperson 418. In some cases, the array of light sensitive elements 402 maybe an array of charge coupled devices (CCD), where each pixel in thearray receives photons, and converts the photons to an analog signal(e.g., voltage or current). Thus, considering the example CCD array as awhole, the CCD array creates a plurality of analog signals indicative ofthe luminance of a portion of the scene or image to which the CCD arrayis exposed. In yet still other cases, the array of light sensitiveelements 402 may be an array of complementary metal-oxide semiconductordevices (CMOS, also known as an Active Pixel Sensor (APS)), where eachpixel in the array receives photons, and converts the photons to ananalog signal. As before, considering the example APS array as a whole,the APS array creates a plurality of analog signals indicative of theluminance of a portion of the scene or image to which the APS array isexposed.

Still referring to FIG. 4, the array of light sensitive elements 402couples to the logarithmic analog-to-digital conversion system 404 byway of a selective coupling network 406. In particular, to capture animage, the analog signal created by each pixel in the array needs to beread and converted to a digital value. The selective coupling network406 thus acts to selectively couple, in some cases sequentially throughthe array, the analog signal created by each pixel to the logarithmicanalog-to-digital conversion system 404. The selective coupling networkmay take any suitable form, such as a single analog multiplexer, or aseries of analog multiplexers. In some cases, the selective couplingnetwork 406 receives commands regarding which pixel in the array oflight sensitive elements 402 to couple to the logarithmicanalog-to-digital conversion system 404 from processor 408. In othercases, the selective coupling network 406 may receive a “start” commandfrom the processor, and sequentially step through the pixels in apredefined order in a predefined period of time.

Logarithmic analog-to-digital conversion system 404 couples on an analogside to the selective coupling network 406, and couples on a digitalside to the processor by way of the communication network 414. Variousexample embodiments of the logarithmic analog-to-digital conversionsystem 404 are discussed in greater detail below, but for now considerthat the system 404 comprises a circuit or combination of circuits suchthat, for each analog signal coupled to the system 404 by way of theselective coupling network 406, the logarithmic analog-to-digitalconversion system 404 produces a digital value that is logarithmicallyrelated to the analog signal. In particular, the logarithmicanalog-to-digital conversion system 404 may create digital values basedon analog input signals according to Equation 3:VALUE_(DIGITAL)=Log_(B)(ANALOG VALUE)+OFFSET  (3)where VALUE_(DIGITAL) is the encoded digital value representation,ANALOG VALUE is the analog signal created by a pixel in the array oflight sensitive elements, B is base of the logarithm, and OFFSET is anoffset value.

Still referring to FIG. 4, processor 408, executing instructions,controls various aspects of the imaging system 400. The processor 408may be any suitable processor, such as a standalone processor, amicrocontroller, a signal processor, state machine, or an applicationspecific integrated circuit (ASIC) specially designed for image captureoperations. For example, in some cases the processor may be a Part No.ATMega328P-PU microcontroller available from Atmel Corporation of SanJose, Calif. In some cases, the instructions executed by the processor408 may be stored in a program memory 410 which may be a non-volatilememory device, such as read-only memory (ROM), electrically erasableprogrammable ROM (EEPROM), or flash memory. The processor may have aworking memory to which programs are copied for execution, or theprocessor may execute the instructions directly from the program memory.

The illustrative imaging system 400 further comprises an image memory412 coupled to the processor 408. As the name implies, the image memory412 may be the location to which digital images are stored. In somecases, the processor 408 may read the digital values from thelogarithmic analog-to-digital conversion system 404 and write the valuesto the image memory, but in other cases the digital values may bedirectly written through a direct memory access (DMA) system. In manycases, the image memory 412 may comprise a removable memory card orstick 420, such that the images captured may be transferred to otherdevices. The image memory 412 may thus comprise any suitable removablememory system or device, such as a Secure Digital (SD) card memory.

Having now described the illustrative imaging system 400, thespecification turns to a description of operation using the logarithmicanalog-to-digital conversion system 404, and how such operationaddresses, at least in part, the issues noted with respect to therelated-art systems. In particular, FIG. 5 shows a graph relating aluminance range (Y-axis on the left) against the range of possibledigital luminance values (X-axis) by way of solid line 500 for anexample 8-bit logarithmic analog-to-digital conversion (thus, thedigital values range from 1 to 256). Co-plotted on the same graph ishuman perception of brightness (Y-axis on the right (with an arbitraryscale of 0 to 100%)) with respect to the digital luminance values(again, the X-axis) by way of dash-dot-dash line 502. Further, the graphof FIG. 5 shows the illustrative octaves 204, 206, 208, 210, 212, 214,216, and 218.

Thus, FIG. 5 illustrates a logarithmic relationship between luminanceand the digital luminance values by way of line 500. FIG. 5 also showsthe human perception of brightness by way of line 502 against the rangeof possible digital luminance values taking into account the logarithmicanalog-to-digital conversion. By performing logarithmicanalog-to-digital conversion, the information regarding luminance isbetter distributed within each illustrative octave. Again consideringhuman perception of brightness, given the logarithmic analog-to-digitalconversion, the change in luminance between a decimal 224 value and adecimal 256 value will be viewed as a doubling of brightness, with 32gradations between them. The difference in brightness as perceived by ahuman of a pixel having a decimal 192 value and a pixel having a decimal224 value will be a doubling of apparent brightness, in this case with32 gradations between. Skipping to the lower octaves, the difference inbrightness as perceived by a human of a pixel having a decimal 32 valueand a pixel having a decimal 64 value will be a doubling of apparentbrightness, with 32 gradations between. Finally, the difference inbrightness as perceived by a human of a pixel having a decimal 1 valueand a pixel having a decimal 32 value will be a doubling of apparentbrightness, with 32 gradations between.

While each illustrative octave represents a doubling of apparentbrightness, FIG. 5 also shows that the granularity or quantizationwithin each octave is evenly distributed about the octaves. Table 3immediately below shows the information of the immediately previousparagraph in table form.

TABLE 3 OCTAVE START VALUE STOP VALUE QUANTIZATION 1 (204) 1 32 32 2(206) 32 64 32 3 (208) 64 96 32 4 (210) 96 128 32 5 (212) 128 160 32 6(214) 160 192 32 7 (216) 192 224 32 8 (218) 224 256 32 Total 256Thus, the various embodiments more evenly distribute the quantizationacross the octaves from a granularity standpoint as related to humanperception. Table 4 shows, for the example eight octaves and 8-bitanalog-to-digital conversion, how the linear and logarithmic conversionscompare.

TABLE 4 QUANTIZATION QUANTIZATION OCTAVE (LINEAR) (LOGARITHMIC) 1 1 32 22 32 3 4 32 4 8 32 5 16 32 6 32 32 7 64 32 8 128 32 Total 256 256With respect to post-processing, in this example there is insufficientdata until one reaches the sixth octave in the linear systems torecreate the granularity in the corresponding octaves in the logarithmicsystem.

Wider Effective Capture Range

The specification now turns to the concepts of dynamic range of capture.An image on a sunny day at the beach, with the image comprising a personperched beneath the shade of an umbrella, has a very wide dynamic rangeof luminance. The example situation of a scene with both beach and shademay have a dynamic range of 160 decibels (dB). By contrast, an imagingsystem with 10-bit linear analog-to-digital conversion theoretically hasa 60 dB capture range (2¹⁰=1024, and 20 log(1024)=60 dB), thoughcurrently available arrays of light sensitive elements may have acapture range approaching, if not meeting, 160 dB. From the study above,however, it is clear that in spite of the theoretical range of a 10 bitlinear analog-to-digital conversion, in actuality the effective range isless as caused by the under quantization in the lower octaves. Table 4above suggests that roughly half the octaves are under quantized, makingthe effective dynamic range of linear analog-to-digital closer to about30 dB. Table 5 shows a relationship between a linear analog-to-digitalconversion and logarithmic analog-to-digital conversion for a 10-bitsystem and 8 octaves to highlight again the effective breadth of theimage capture for wider conversion systems.

TABLE 5 QUANTIZATION QUANTIZATION OCTAVE (LINEAR) (LOGARITHMIC) 1 1 1282 8 128 3 16 128 4 32 128 5 64 128 6 128 128 7 256 128 8 512 128 Total1024 1024If you consider that somewhere between 64 and 128 quantization stepswithin each octave is the subjective point where degradation becomesnoticeable to an ordinary viewer, Table 5 thus highlights again that theeffective dynamic range for the linear conversion is about half(arguably less than half given the 64 quantization steps in illustrativezone 5) that of the logarithmic conversion.

In Camera Contrast Control

The specification now turns to contrast and brightness control inaccordance with at least some embodiments. As an aid in discussing theconcepts of contrast and brightness control, attention is directed toFIG. 6 which plots digital luminance values (Y-axis) against luminanceacross the entire visual range (X-axis in a logarithmic scale). Inparticular, dashed line 600 plots the relationship between digitalluminance values against luminance if the bit width of the digitalluminance values was sufficient to capture the entire visual luminancerange. However, for purposes of this portion of the discussion, assumethat the bit-width of the digital luminance values is insufficient tocapture the entire luminance range. In particular, consider that the bitwidth of the digital luminance values is limited to the magnitude ofrange 604 of solid line 602. Various camera controls may be implementedto shift line 602 as desired by the camera operator. For example, bycontrolling shutter speed (exposure time of the scene to the array oflight sensitive elements) the position of line 602 may be shiftedparallel to dashed line 600. For example, slower shutter speedsresulting longer exposure time make the resultant image more sensitiveto the lower luminance ranges (i.e., shift line 602 down the slope ofdashed line 600), with the higher luminance ranges potentiallyoverexposed. Likewise, faster shutter speeds resulting in shorterexposure time make the resultant image less sensitive to the lowerluminance ranges (i.e., shift the line 600 up the slope of dashed line600), with the lower luminance ranges potentially underexposed.

However, in accordance with at least some embodiments additionalcontrols may be implemented, the additional controls in the form ofcontrast control and brightness control. FIG. 7 shows, in block diagramform, the imaging system 400 in greater detail in some respects, andwith additional illustrative components. In particular, FIG. 7 shows anillustrative implementation of the logarithmic analog-to-digitalconversion system 404, as well as a display device 700 comprising atouchscreen overlay 702 (as shown by a raised corner), and externallyaccessible switches 704. Each will be addressed in turn, and then thefunctionality of contrast and brightness control in relation to thevarious devices will be discussed.

The logarithmic analog-to-digital conversion system 404 in FIG. 7 isillustratively implemented as an amplifier 706 coupled on an input sideto the array of light sensitive devices 402 and coupled on an outputside to an analog-to-digital converter 708. The amplifier 706 inaccordance with these embodiments has a gain response that islogarithmic (e.g., Part No. AD8304, available from Analog Devices, Inc.of Norwood, Mass.), and the analog-to-digital converter 708 has a linearresponse (e.g., Part No. LTC2480 16-bit A/D converter available fromLinear Technologies of Milpitas, Calif.). That is, the output signal ofthe amplifier 706 is logarithmically related to the input signal.Amplifier 706 may further have control inputs, such as a gain control710 and a bias or offset control 712. In some cases, the gain control710 and offset control 712 are analog inputs, and in the illustrativeimaging system the gain control and offset control are coupled to adigital-to-analog output portion 714 of the processor 408 (such as whenthe processor 408 is a microcontroller or ASIC). In other cases, theanalog values may be created by a digital-to-analog converter distinctfrom the processor 408, yet communicatively coupled to both theprocessor 408 and the amplifier 706. In other cases, the gain and offsetof the amplifier 706 may be controlled by way of a digital controlsignal or signals, and thus the processor 408 may couple to theamplifier by way of a digital communication bus. Regardless of theprecise mechanism by which the processor, executing instructions,controls the gain and/or offset of the amplifier, how such controlaffects the operation of the imaging system is discussed afterdiscussing the display device 700.

The example imaging system 400 of FIG. 7 further comprises a displaydevice 700 coupled to the processor 408 such that various images andinterfaces may be displayed. The display device 700 may be any suitabledisplay device, such as a liquid crystal display (LCD) or plasmadisplay. In some embodiments, the display device 700 may be overlaidwith a touchscreen overlay 702 (e.g., a capacitive touch screen overlay)such that a user of the imaging system 400 can interact with theinstructions executing on the computer system by way of the touchscreenoverlay 702 and display device 700. As an example of such interaction,the example display device 700 is shown to display two slider bars 716and 718. Thus, by interacting with the slider bars (such as by movingblock 720 up or down) various functionalities may be implemented, suchas changing brightness. Likewise, by interacting with the slider bars(such as by moving block 722 left or right) various functionalities maybe implemented, such as changing contrast.

However, in other cases the user may interact with the programsexecuting on the processor by way of physical buttons accessible on theoutside cover 724 of the imaging system, such as illustrative externallyaccessible switches 704. In particular, the illustrative externallyaccessible switches 704 couple to the processor 408 by way of digitalinputs 726 of the processor 408 (such as when the processor 408 is amicrocontroller or ASIC). In other cases, the digital values may be readby a digital input device distinct from the processor 408, yetcommunicatively coupled to both the processor 408 and the externallyaccessible switches 704. While illustrative externally accessibleswitches 704 are shown as two normally open pushbutton devices, othertypes and number of externally accessible switches may be used.

Referring to FIGS. 6 and 7 simultaneously, in accordance with at leastsome embodiments, the processor (executing instructions) selectivelycontrols contrast and/or brightness. For example, by controlling thesignal provided to the offset control 712, the processor may controlbrightness of the recorded scene. The brightness control in relation toFIG. 6 is a shifting of line 602 up or down. For example, by loweringthe offset of the amplifier 706, the relationship of the digitalluminance values to the luminance may be shifted downward, as shown bydashed line 606. Likewise, by raising the offset of the amplifier 706,the relationship of the digital luminance values to the luminance may beshifted upward (not specifically shown). Notice how the magnitude of therange 604 stays the same in spite of the shifting (i.e., the projectionof the line 602 against the Y-axis has range 604, and the projection ofthe line 606 against the Y-axis has a range of the same magnitude).Thus, a relatively dark scene may be captured “lighter” if desired, anda relatively bright scene may be captured “darker” if desired.

Further, by controlling the signal provided to the gain control 710, theprocessor may control contrast of the recorded scene. The contrastcontrol in relation to FIG. 6 is a changing of the slope of line 602 tobe greater (closer to vertical, or lesser (closer to horizontal).Considered more mathematically, and considering again Equation (3)above, changing the contrast is effectively a change in the base B ofthe logarithm conversion. For example, by lowering the gain of theamplifier 706, the relationship of the digital luminance values to theluminance may be shifted such that a greater luminance range is capturewithin the range of the digital luminance values, as shown bydashed-dot-dash line 608. Such a shift results in fewer gradationswithin each octave (the octaves not specifically shown). Likewise, byraising the gain of the amplifier 706, the relationship of the digitalluminance values to the luminance may be shifted such that a lesserluminance range is captured within the range of the digital luminancevalues (not specifically shown). Such a shift results in a greaternumber of gradations within each octave. Notice how the magnitude of therange 604 stays the same in spite of the shifting (i.e., the projectionof the line 602 against the Y-axis has range 604, and the projection ofthe dash-dot-dash line 608 against the Y-axis has a range of the samemagnitude). Thus, a scene with a relatively narrow dynamic range (e.g.,cloudy day) may be captured with greater quantization within eachoctave, if desired. Likewise a relatively bright scene (e.g., sunny day)may be more broadly captured across the dynamic range of the scene withlesser quantization within each octave, if desired.

In some cases, the contrast and/or brightness control is implementedbased on commands received from the user of the imaging system 400. Forexample, as a precursor to capturing an image for storage to the imagememory, the imaging system 400 may be configured to capture images anddisplay the images on the display device (e.g., a view-finderfunctionality). As part of capturing the initial image, the imagingsystem 400 may enable the user to make contrast and brightness controladjustments, such as by interacting with the slider bars 716 and/or 718,by interacting with the externally accessible switches 704, or both.Interacting with the interface devices results in changes to the gainand offset (i.e., contrast and brightness, respectively). Once the userhas adjusted contrast and/or brightness as desired, the final image maybe captured and stored to the image memory 412.

In yet still other embodiments, adjustments to contrast and/orbrightness may be made by a program executing on the processor 408without user input. In particular, by performing logarithmicanalog-to-digital conversion an automatic system for contrast and/orbrightness may be implemented. In the “automatic” adjustment example,the imaging system 400 may capture an initial image (that may or may notbe shown on the display device) with each of the contrast and brightnessat a predefined “center” or midrange setting. The processor 408 mayanalyze the initial image to determine the dynamic range of the image,and may adjust the contrast and/or brightness based on the dynamicrange, again without user input. For example, the processor 408 maylocate within the initial image the pixel having the lowest digitalluminance value, and the pixel having the highest digital luminancevalue. An initial key value (e.g., average of the highest and lowestluminance values) may be determined, and the brightness (offset) may beadjusted such that the key value of the final image resides at or near apredetermined value. Note that as discussed with respect to FIG. 3above, related-art systems using linear analog-to-digital conversioncould not use the mathematical center of the digital luminance values,as the average or mathematical center in those systems turns out toreside in the upper luminance octaves. As yet another example, theprocessor 408 may locate within the initial image the pixel having thelowest digital luminance value and the pixel having the highest digitalluminance value, and from the determined digital luminance valuescalculate a dynamic range (e.g., based on a difference between thehighest and lowest digital luminance values). The contrast (gain) maythen be adjusted such that the dynamic range of the analog-to-digitalconversion more closely matches the dynamic range of the scene to becapture for the final image, again without user input.

Full Range Capture

The adjustments to the contrast and/or brightness work for any bit widthof the digital luminance values; however, adjustments may be performedwith greater accuracy if the entire dynamic range of the scene iscaptured in the initial image. Performing logarithmic analog-to-digitalconversion enables capturing the entire dynamic range of a scene with arelatively narrow bit width analog-to-digital conversion. Consider,again, a beach scene with a person perched beneath an umbrella, and thusthe scene spanning approximately the entire 160 dB visual range of humanperception. If an 11-bit logarithmic analog-to-digital conversion isperformed, 2048 total gradations are possible. If the illustrative 160dB scene is logically broken into 10 octaves, each octave thus has justover 200 gradations or quantization steps. Further considering that“good” image reproduction is possible with 128 or more gradations orsteps within each octave, the entire 160 dB scene captured with an11-bit logarithmic analog-to-digital conversion gives better than “good”granularity—200 quantization steps per octave—across the entire 160 dBscene. Thus, in some example systems, no contrast and/or brightnesscontrol is needed as the imaging system captures the entire dynamicrange every time.

However, not every image capture will need to have a dynamic rangespanning the entire 160 dB visual range of human perception. In thebeach example, the user may—for style reasons only—want to capture theperson perched beneath the umbrella and may not want high fidelity imagecapture of the surf behind the person. In the case of scenes illuminatedwith indoor lighting, the dynamic range of the scene may besignificantly less than the 160 dB. However, the ability to at leastinitially capture the entire 160 dB visual range of human perception, orat least ability to capture a dynamic range than wider than the actualscene, may be helpful in performing contrast and/or brightness control.Consider, as an example, that the processor 408 captures an initialimage, and from the initial image determines that the scene spanssomething less than the entire 160 dB range of the array of lightsensitive devices and/or logarithmic analog-to-digital conversion. Theprocessor may then adjust the contrast (gain) and the brightness (span)such that capture range of the logarithmic analog-to-digital conversionis co-extensive with the scene. Stated otherwise, with the contrast andbrightness set such that the at least the entire dynamic range of ascene can be capture within the initial image, the processor 408 mayanalyze the initial image (which may or may not be shown on the displaydevice 700) to determine the darkest digital luminance value and thelightest digital luminance value, and adjust the contrast and gain suchthat the adjusted dynamic range of the logarithmic analog-to-digitalconversion matches that of the actual scene (or perhaps is slightlylarger). The result is that the number of gradations or quantizationsteps within each octave of the captured image is higher than the fullrange image capture.

It follows from the discussion that many of the parameters ofphotography can be eliminated from the camera. The mechanical shutterand shutter speed control (whether mechanical or electro-mechanical) arenot strictly required in digital cameras, as the exposure time controlcan be implemented electronically in how long the array of lightsensitive devices elements are exposed to a scene after reset and beforethe analog signals are read. High end digital cameras may implementmechanical shutters and shutter speed control as a mechanism to supplyvisual effects to an image, but in other systems the mechanical shutterand shutter speed controls may be omitted such that the imaging systemdoes not have a mechanical shutter or mechanical shutter speed control.It is noted that an electronic “shutter” (e.g., zeroing the CCD/APSarray voltages just prior to capture of the image) may still be used.Likewise, aperture control need not be present, but may find use inhigher-end cameras to facility certain visual effects in the captureimages. Moreover, the idea of “film speed” or ISO is eliminated,particularly in systems where full range image capture is performedand/or the contrast and/or brightness are adjustable to match the visualrange of the scene. Stated otherwise, some imaging system in accordancewith at least some embodiments do not implement a “film speed” or ISOcontrol.

Other Example Linear A/D Conversion Systems

The specification to this point has expressly illustrated a logarithmicanalog-to-digital conversion, and shown one possible implementation of alogarithmic analog-to-digital conversion in FIG. 7. However, furtherlogarithmic analog-to-digital conversion systems are also possible. FIG.8 shows a circuit diagram of another example embodiment of a logarithmicanalog-to-digital conversion system 404 in accordance with furtherembodiments. In particular, FIG. 8 shows a linear amplifier 706 (e.g.,Part No. LMC6001 amplifier available from Texas Instruments, Inc. ofDallas, Tex.) coupled on an input side to the array of light sensitivedevices 402 (not specifically shown in FIG. 8) and coupled on an outputside to a logarithmic analog-to-digital converter 802. The amplifier 800in accordance with these embodiments has a gain response that is linear(see equation (1) above and related discussion). That is, the outputsignal of the amplifier 800 is linearly related to the input signal.Amplifier 706 may further have control inputs, such as a gain control804 and a bias or offset control 806. In some cases, the gain control804 and offset control 806 are analog inputs coupled to adigital-to-analog output portion of the processor 408 (not specificallyshown in FIG. 8). In other cases, the analog values may be created by adigital-to-analog converter distinct from the processor 408, yetcommunicatively coupled to both the processor 408 and the amplifier 800.In other cases, the gain and offset of the amplifier 800 may controlledby way of a digital control signal or signals, and thus the processor408 may couple to the amplifier by way of a digital communication bus.Regardless of the precise mechanism by which the processor, executinginstructions, controls the gain and/or offset of the amplifier, the gainand offset may be controlled as discussed above.

The logarithmic analog-to-digital converter 802 in accordance with theseembodiments has a response that is logarithmic. That is, the digitaloutput values created by the logarithmic analog-to-digital converter 802are logarithmically related to the input signal. Thus, the combinationof the linear amplifier 800 and logarithmic analog-to-digital converter802 may be used to implement some or all the various embodimentsdiscussed above.

Still referring to FIG. 8, in further cases, a logarithmicanalog-to-digital converter 802 may itself have a gain input signal 808and/or an offset signal 810, which, if present, couple to the processor408 similarly to the gain and offset discussed with the respect toamplifier 706 and 800. Thus, the contrast (gain) control may beimplemented in whole or in part by controlling the gain of thelogarithmic analog-to-digital converter 802, and the brightness (offset)control may be implemented in whole or in part by controlling the offsetof the logarithmic analog-to-digital converter 802.

Color Systems

The various embodiments discussed to this point have been in relation toblack and white capture systems so as not to unduly complicate thedescription. However, the various embodiments are equally applicable tocolor systems. In particular, color systems may be implemented in avariety of ways. For example, in some cases in a grouping of lightsensitive elements (e.g., three abutting light sensitive elements in thearray) each light sensitive element may be associated with a colorfilter such that each light sensitive element is responsive to a singlecolor of light. One light sensitive element may be responsive only tored light, one light sensitive element may be responsive only to greenlight, and light sensitive element may be responsive only to blue light.Thus, the luminance values in this example are each associated with aparticular color.

In other cases, multiple arrays of light sensitive elements 402 may bepresent, with a set of optics such that each array is exposed to thescene, and each array is filtered for a single color (e.g., red, green,and blue). In the multiple array case, there may be multiple logarithmicanalog-to-digital conversion systems 404, one each for each color. Inyet still other cases, the array of light sensitive elements 402 may bea Bayer array comprising a pseudo random distribution of colorsensitivity (e.g., a small dot of filter material over each lightsensitive element to make the element sensitive to only one color). Thelogarithmic analog-to-digital conversion discussed in this specificationworks equally well in color systems.

File Format

In accordance with at least some embodiments, each digital value createdby the logarithmic analog-to-digital conversion system 404 may be storedto the image memory 412, such as a series of digital values startingwith a pixel in the upper left corner of the scene, and working alongeach row until reaching the lower right hand corner of the scene. In thecase of color images, additional bits may be included to designate thecolor associated with each digital value. For example, for a 10-bit widelogarithmic analog-to-digital conversion system, each digital value maybe 10 bits, with an associated color 2-bit color indication, for a totalof 12 bits for each pixel.

However, in other embodiments the file format may take advantage of theuse of octaves. In particular, in some embodiments the digital values inthe image memory 412 are stored as a value indicative of octave (whichmay also be referred to as a radix), a value indicative of graduation orquantization within the octave (the graduation may also be referred toas a mantissa), and a value indicative of a color. Consider, forexample, a 10-bit logarithmic analog-to-digital conversion having eightoctaves. As discussed with respect to Table 5, there are 128 graduationsor quantization steps within each of the illustrative octaves. Storingthe digital values in this example thus involves, for each digitalluminance value, storing a 3-bit indication of octave (2³=8), a 7-bitindication of graduation or quantization within the octave (2⁷=128), anda 2-bit indication of color.

Many times particular areas within an image (e.g., along a portion ofthe same horizontal scan line) may have luminance values within the sameoctave. By storing digital values based on octave and gradation withinthe octave, for groups of pixels within the same octave the bits relatedto octave may be omitted. For example, consider a file storage formatwhere, as a default, each digital value comprises a value indicative ofcolor, followed by a value indicative of octave, and then followed by avalue indicative of gradation within the octave. For systems using red,green, and blue, of the four states for the two bits designated for thevalue indicative of color, one state is unused (e.g., if binary 00designates red, binary 01 designates blue, and binary 10 designatesgreen, and thus binary 11 is unused). In the example system, when aseries digital luminance values all reside within the same octave, theunused “color” value may be inserted into the file along with anindication of the octave, and an indication of the number of subsequentpixels to which the octave applies. For the next number of designatedpixels, only the color bits and the mantissa may be written to the file,omitting the octave.

Consider a more specific example of omitting the values indicative ofoctave. In particular, consider a scene comprising the horizon on asunny, blue sky day with no clouds. Further consider that in the exampleimage that the top scan line of an image is all blue, and that theshades of blue on the scan line all reside in the same octave. In theexample system, the programs writing the image to the image memory maydetermine that the entire line of pixels resides within the same octave.The first value written may be a “repeat” indication (e.g., a value 11in the “color” designation) followed by an indication of octave and anumber of predetermined bit-width indicative of how many pixels thatfollow reside within the same octave. If the scan line is 1024 pixelswide, the predetermined number may be 10 bits wide. The next digitalvalue may omit the value indicative of octave.

Piecewise Logarithmic A/D Conversion

The various embodiments discussed to this point have assumed a singleanalog-to-digital converter that directly, or in combination with othercomponents, performs logarithmic analog-to-digital conversion. However,in other embodiments the logarithmic analog-to-digital conversion may bea piecewise logarithmic analog-to-digital conversion where no specificcomponent has a logarithmic relationship input-to-output, but as wholecreates an approximation of a logarithmic analog-to-digital conversion.

FIG. 9 shows, in block diagram form, the imaging system 400 withalternative illustrative components. In particular, FIG. 9 shows anillustrative implementation of the logarithmic analog-to-digitalconversion system 404 (here in the form of a piecewise logarithmicanalog-to-digital conversion), as well as a representative grouping ofother components previously discussed. Components previously discussed,and whose operation does not change with respect to previousembodiments, will not be reintroduced here.

The logarithmic analog-to-digital conversion system 404 in FIG. 9 is apiecewise conversion illustratively implemented as an amplifier 900coupled on an input side to the array of light sensitive devices 402 andcoupled on an output side to an illustrative two linearanalog-to-digital converters 902 and 904. The amplifier 900 inaccordance with these embodiments has a gain response that is linear. Tochange the operational range of each linear analog-to-digital converter,a linear amplifier 906 and 908 are coupled between amplifier 900 and therespective linear analog-to-digital converters 902 and 904. Amplifier900 may further have control inputs, such as a gain control 910 and abias or offset control 912. The gain control 910 and offset control 912may be either analog input values or digital input values. In theexample case of analog input values, the gain control and offset controlare coupled to a digital-to-analog output portion 714 of the processor408 (such as when the processor 408 is a microcontroller or ASIC). Inother cases, the analog values may be created by a digital-to-analogconverter distinct from the processor 408, yet communicatively coupledto both the processor 408 and the amplifier 706. The processor(executing instructions) selectively controls contrast and/or brightnessas discussed above, either automatically, or by interaction with theuser.

In order to discuss the piecewise logarithmic analog-to-digitalconversion of the embodiments of FIG. 9, attention is directed to thegraph of FIG. 10. In particular, FIG. 10 relates a luminance range(Y-axis on the left) against the range of possible digital luminancevalues (X-axis) by way of two solid lines 1000 and 1002, where therelationship is piecewise logarithmic (with a true logarithmicrelationship shown in by solid line 1004). Co-plotted on the same graphis human perception of brightness (Y-axis on the right (with anarbitrary scale of 0 to 100%)) with respect to the digital luminancevalues (again, the X-axis) by way of dash-dot-dash line 1006.

In accordance with these example embodiments, the amplifiers 906 and 908are adjusted such that their respective linear analog-to-digitalconverters are particularly sensitive to the depicted ranges ofluminance. That is, for example, linear analog-to-digital converter 902may be configured by way of amplifier 906 to be particularly sensitiveto range 1008 (the lower luminance values, line 1000), and linearanalog-to-digital converter 904 may be configured by way of amplifier908 to be particularly sensitive to range 1010 (the upper luminancevalues, line 1002). To create the digital luminance values written tothe image memory, for each element in the array of light sensitiveelements the processor 408, executing instructions, reads both thelinear analog-to-digital converters 902 and 904. In this example, whenthe processor determines the luminance value is above a predeterminedthreshold (i.e., the luminance is above a certain octave, in thisexample in octave 214 or above), the processor records the digitalluminance value from the linear analog-to-digital converter 904.Likewise, when the processor determines the luminance value is below apredetermined threshold (i.e., the luminance is below a certain octave,in this example in octave 212 or below), the processor records thedigital luminance value from the linear analog-to-digital converter 902.Stated otherwise, a first portion of the plurality of digital luminancevalues are read from the first linear analog-to-digital converter, and asecond portion of the plurality of digital luminance values are readfrom the second linear analog-to-digital converter. In some cases, foroverlap octaves (e.g., overlap octave 212 in the example of FIG. 10),the processor may choose either value, or combine the values in some way(e.g., averaging).

Though FIGS. 9 and 10 discuss only two linear analog-to-digitalconverters, it will be understood that three or more linearanalog-to-digital converters may be used, and that better piecewiseapproximation of a logarithmic analog-to-digital conversion may takeplace. Likewise, in other embodiments a single linear analog-to-digitalconverter may be used, reading a first exposure at a first setting(e.g., lines 1000 in FIG. 10), and then reading a second exposure at asecond setting (e.g., line 1002 of FIG. 10). While such a system may beoperable for still life pictures, such may not be suitable for imageswhere significant movement is present.

FIG. 11 shows a method in accordance with at least some embodiments. Themethod may be performed, in whole or in part, by instructions executingon a processor. In particular, the method starts (block 1100) andproceeds to capture an image (block 1102) by: reading a plurality ofanalog signals from an array of light sensitive elements (block 1104);performing logarithmic analog-to-digital conversion on the plurality ofanalog signals which creates a corresponding plurality of digital values(block 1106); and storing representations of the plurality of digitalvalues to a storage medium (block 1108). Thereafter, the method ends(block 1110), possibly to be restarted. The performing logarithmicanalog-to-digital conversion on the plurality of analog signals whichcreates a corresponding plurality of digital values may take many formsusing different types of physical devices, and includes cases wherepiecewise logarithmic analog-to-digital conversion is performed.

Audio Logarithmic A/D Conversion

The various embodiments discussed to this point have been in relation tocapturing images of visual scenes; however, the logarithmicanalog-to-digital conversion likewise finds use in capturing audio, suchas associated with a video camera capturing a scene. In particular,human hearing is likewise logarithmic in nature, yet audio capturesystems use linear analog-to-digital conversion to capture audio. Theresult is that higher amplitude (i.e., higher volume) “scenes” areover-sampled in the same way as discussed above for visual images, andlower amplitude (i.e., lower volume) “scenes” are under-sampled in thesame way discussed above. Thus, all the various embodiments discussedabove may be extended to audio capture using a single or multiplemicrophones, the audio capture either concurrent with visual imagecapture, or standing alone. Moreover, the file storage format discussedabove (less the color bits) may also be used to record the audio imageto the image memory, with the understanding that what is stored for eachaudio channel is a time series of sample audio signals rather than acomplete set of luminance values at the “same” point in time.

From the description provided herein, those skilled in the art arereadily able to combine software created as described with appropriategeneral-purpose or special-purpose computer hardware to create acomputer system and/or computer sub-components in accordance with thevarious embodiments, to create a computer system and/or computersub-components for carrying out the methods of the various embodiments,and/or to create a non-transitory computer-readable storage medium(i.e., other than an signal traveling along a conductor or carrier wave)for storing a software program to implement the method aspects of thevarious embodiments.

References to “one embodiment,” “an embodiment,” “some embodiments,”“various embodiments”, “example embodiments” or the like indicate that aparticular element or characteristic is included in at least oneembodiment of the invention. Although the phrases may appear in variousplaces, the phrases do not necessarily refer to the same embodiment.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. For example, the types of camerasto which the various example embodiments may be applied comprise SLRcameras, point and shoot cameras, scientific cameras, medical cameras,automotive cameras, personal computer cameras, video cameras, andsurveillance cameras. Also, it would be possible to invert thelogarithmic conversion such that the highest digital values encode thedarkest portions of the scene. It is intended that the following claimsbe interpreted to embrace all such variations and modifications.

What is claimed is:
 1. A method comprising: capturing an image byreading a plurality of analog signals from an array of light sensitiveelements; performing logarithmic analog-to-digital conversion on theplurality of analog signals which creates a corresponding plurality ofdigital values, each corresponding to a digital luminance value; andstoring representations of the plurality of digital values to a storagemedium; prior to the capturing the image: adjusting brightness bychanging offset applied to the analog signals prior to analog-to-digitalconversion; and adjusting contrast by changing gain applied to theanalog signal prior to analog-to-digital conversion, wherein prior toadjusting brightness and adjusting contrast, the method furthercomprises capturing an image to create an initial image; and calculatinga dynamic range regarding the initial image, the calculating performedby a processor executing a program; and wherein adjusting the brightnessfurther comprises adjusting the brightness based on the dynamic range,the adjusting the brightness performed by the processor without userinput; and wherein adjusting the contrast further comprises adjustingthe contrast based on the dynamic range, the adjusting the contrastperformed by the processor without user input.
 2. The method of claim 1wherein performing logarithmic analog-to-digital conversion furthercomprises: applying each analog signal to an amplifier whose gainresponse is logarithmic to create a modified signal; and then applyingthe modified signal to a linear analog-to-digital converter.
 3. Themethod of claim 1 wherein performing logarithmic analog-to-digitalconversion further comprises: performing piecewise logarithmicanalog-to-digital conversion by sequentially applying each analog signalto a first linear analog-to-digital converter and simultaneouslyapplying to a second linear analog-to-digital converter distinct fromthe first linear analog-to-digital converter; reading a first portion ofthe plurality of digital values from the first linear analog-to-digitalconverter; and reading a second portion of the plurality of digitalvalues from the second linear analog-to-digital converter.
 4. The methodof claim 1 wherein adjusting the brightness further comprises adjustingthe brightness by a user by way of an interface device; and whereinadjusting the contrast further comprises adjusting the contrast by theuser by way of an interface device.
 5. The method of claim 1 whereinstoring representations of the plurality of digital values furthercomprises storing, for each digital value, a combination of a valueindicative of an octave and a value indicative of gradation within theoctave.
 6. The method of claim 1 wherein storing representations of theplurality of digital values further comprises storing, for at least onedigital value, a combination of a value indicative of an octave, a valueindicative of gradation within the octave, and a value indicative of acolor.
 7. The method of claim 1 wherein capturing the image furthercomprises capturing by at least one selected from the group consistingof: a mobile device; a mobile cellular device; a scanner; a copier; anda digital camera.
 8. The method of claim 1 wherein reading furthercomprising reading from at least one selected from the group consistingof: an array of charge coupled devices (CCD); an array of complementarymetal oxide semiconductor (CMOS) devices.
 9. A system comprising: aprocessor; a program memory coupled to the processor, the program memorybeing a non-transitory computer-readable storage medium; an array oflight sensitive elements; a means for performing logarithmicanalog-to-digital conversion that defines an analog side and a digitalside, the means for performing coupled to the processor on the digitalside; and an image memory coupled to the processor; wherein the programmemory stores a program that, when executed by the processor, causes theprocessor to capture an image by causing the processor to: sequentiallycouple each light sensitive element of the array to the analog side ofthe means for performing; and concurrently read a plurality of digitalvalues corresponding to a signal associated with each light sensitiveelement; and store a representation of the plurality of digital valuesto the image memory; wherein the means for performing comprises anamplifier that defines an offset adjustment and a gain adjustment; andwherein prior to capturing the image, the program further causes theprocessor to: capture an initial image using the array of lightsensitive elements and the means for performing; determine a dynamicrange of values in the initial image; and at least one selected from thegroup consisting of—modify the offset adjustment of the amplifier basedon the dynamic range of values without input from a user, and modify thegain adjustment of the amplifier based on the dynamic range of valueswithout input from the user.
 10. The system of claim 9 wherein theamplifier produces an analog output signal logarithmically related to ananalog input signal; and a linear analog-to-digital converter coupled tothe analog output signal of the amplifier.
 11. The system of claim 9wherein the amplifier produces an analog output signal linearly relatedto an analog input signal; and a logarithmic analog-to-digital convertercoupled to the analog output signal of the amplifier.
 12. The system ofclaim 9 wherein the means for performing further comprises: a firstlinear analog-to-digital converter coupled to the amplifier; and asecond linear analog-to-digital converter coupled amplifier, the secondlinear analog-to-digital converter distinct from the first linearanalog-to-digital converter; wherein the processor reads a plurality ofdigital values corresponding to a signal associated with each lightsensitive element, the program further causes the processor to: read afirst portion of the plurality of digital values from the first linearanalog-to-digital converter; and read a second portion of the pluralityof digital values from the second linear analog-to-digital converter.13. The system of claim 9 further comprising: a display device coupledto the processor; wherein the program further causes the processor to:using; display the initial image on the display device; and at least oneselected from the group consisting of—modify the offset adjustment ofthe amplifier based on a command from a user, and modify the gainadjustment of the amplifier based on a command from the user.
 14. Thesystem of claim 9 further comprising: a display device coupled to theprocessor; a touch sensitive overlay associated with the display device,and the touch sensitive overlay communicatively coupled to theprocessor; wherein the program further causes the processor to: displaythe initial image on the display device; and at least one selected fromthe group consisting of—modify the offset adjustment of the amplifierbased on a command from a user received through the touch sensitiveoverlay, and modify the gain adjustment of the amplifier based on acommand from the user received through the touch sensitive overlay. 15.The system of claim 9 wherein when the processor stores therepresentation of the plurality of digital values to the image memory,the program causes the processor, for at least one digital value, to:store a first value in the image memory, the first value indicative ofan octave; and store a second value in the image memory, the secondvalue indicative of a gradation within the octave.
 16. The system ofclaim 15 wherein when the processor stores the representation of theplurality of digital values to the image memory, the program causes theprocessor, for each digital value, to store a value indicative of acolor associated with the digital value.
 17. The system of claim 9wherein the processor, program memory, array of light sensitiveelements, logarithmic analog-to-digital converter, and image memory arepart of at least one selected from the group consisting of: a mobilecellular device; a digital camera; a scanner; a copier; and a digitalcamera.
 18. The system of claim 9 wherein the array of light sensitivedevices is at least one selected from the group consisting of: an arrayof charge coupled devices (CCD); an array of complementary metal oxidesemiconductor (CMOS) devices.